Kinetic data on the oxidation and oxygen chemical diffusion of uranium-plutonium oxides is important in assessing the chemical behaviour of the oxide. The oxidation of uranium-plutonium oxides, both hypostoichiometric [(U,Pu)O2−x] and hyperstoichiometric [(U,Pu)O2+x], with varying plutonium content, was studied by non-isothermal methods of thermogravimetry. The kinetics of the oxidation were found to be controlled by oxygen chemical diffusion, with activation energies in the range 7–12 kcal mole−1 for (U, Pu)O2−x, and 15–19 kcal mole−1 for (U, Pu)O2+x. The effect of variation of plutonium content on the kinetics was negligible.